1. Field of the Invention
This invention relates to an apparatus and a method for monitoring tension in optical fibres, in particlar during the drawing of such fibres. More generally the invention relates to a method of manufacturing optical fibres.
2. Description of the Related Art
During the process of drawing optical fibres from a preform a number of parameters, which influence the quality of the final product, need to be controlled. The heating temperature of the preform and the drawing rate must be in balance so that the fibre can be continuously drawn under uniform conditions. This is normally accomplished by continuous on-line monitoring of the tension within the fibre. Furnace temperature may be adjusted to reduce or increase the drawing tension. Among the existing methods for on-line measurement of the fibre tension, those that do not involve a physical contact with the fibre are often preferred.
Japanese patent application No. 57-196737 describes a contactless-measuring system for detecting the tension of an optical fibre during drawing. A circularly polarised light beam passes through an optical fibre. The light that has passed through the fibre is subjected to birefringence due to the tension which is being imposed on the fibre. As acknowledged in the application, the described system enabled to detect tensions from 5 to 200 g in optical fibres of 125 μM diameter and that measurements were possible at drawing speed up to 9 m/s.
P. L. Chu, T. Whitbread and P. M. Allen in “An on-line fiber drawing tension and diameter measurement device”, Journal of Lightwave Technologies, vol. 7 pg. 255 (1989) describes a non-contact tension meter to monitor on line the drawing tension. In the disclosed tension meter polarised light illuminates the fibre laterally. The built-in thermal stress and the applied drawing tension transforms the fiber into an anisotropic structure so that the polarised light after traversing through the fibre cross-section suffers a retardation. The direction of polarisation is set at 45° to the fibre axis so that the components of the light polarised parallel and perpendicular to the fibre axis are equal. FIG. 1 illustrates the ray trajectory through the fibre cross-section. When the ray emerges from the fibre, there is a phase shift between the two components, which is called retardation R. The retardation is a function of the ray incident position y and is given by:
                              R          ⁡                      (            y            )                          =                  2          ⁢          C          ⁢                                    ∫              y              b                        ⁢                                                                                σ                    z                                    ⁡                                      (                    r                    )                                                  ⁢                r                ⁢                                  ⅆ                  r                                                                                                  r                    2                                    -                                      y                    2                                                                                                          (        1        )            where C is the stress-opto coefficient of the fiber and the variables y, r, b are shown in FIG. 1. The axial stress σz(r) has two components, the applied drawing tension T and the built-in thermal stress σz0(r):σz(r)=T/πb2+σz0(r).  (2)The contribution due to the built-in thermal stress is considered to be small compared to the contribution due to the applied tension.The axial stress can be derived from Eq. (1) by solving the integral equation:
                                          σ            z                    ⁡                      (            r            )                          =                                            -              1                                      π              ⁢                                                          ⁢              C                                ⁢                                    ∫              r              b                        ⁢                                                                                ⅆ                                          R                      ⁡                                              (                        y                        )                                                                              /                                      ⅆ                    y                                                                                                              y                      2                                        -                                          r                      2                                                                                  ⁢                                                          ⁢                              ⅆ                y                                                                        (        3        )            Thus, if the retardation function R(y) of the fibre is known, the axial stress profile can be reconstructed through Eq. (3).
The light scattered from the fibre is collected by two photodetectors after passing a quarter-wave plate and polarisers. There are two polarisers at the detection side: one is set at +45° and the other polariser is set at −45° to the principal axis. The detected voltages resulting from the two polarisers are subtracted giving an output proportional to the fibre drawing tension. If the input ray is fixed to y values between y1 and y2, the power on the detector is given by:P0=∫y1y2I cos2(R(y)/2−β)dy,  (4)where I is the intensity of the optical beam and β is the angle from the quarter-wave plate axis. Using two detectors, detectors 1 and 2, with β set to −45° and +45° for detector 1 and 2 respectively, the subtraction of the powers on the two detectors:
                                                                                          P                  2                                -                                  P                  1                                            =                            ⁢                                                ∫                                      y                    1                                                        y                    2                                                  ⁢                                  ❘                                                            [                                                                        cos                          2                                                ⁡                                                  (                                                                                                                    R                                ⁡                                                                  (                                  y                                  )                                                                                            /                              2                                                        +                                                          45                              ⁢                              °                                                                                )                                                                    ]                                        -                                                                  [                                                                              cos                            2                                                    ⁡                                                      (                                                                                                                            R                                  ⁡                                                                      (                                    y                                    )                                                                                                  /                                2                                                            -                                                              45                                ⁢                                °                                                                                      )                                                                          ]                                            ⁢                                              ⅆ                        y                                                                                                                                                                    =                            ⁢                              ❘                                                      ∫                                          y                      1                                                              y                      2                                                        ⁢                                      sin                    ⁡                                          (                                              R                        ⁡                                                  (                          y                          )                                                                    )                                                                                                                              (        5        )            For small R(y), P2−P1 can be assumed to be linear with R(y):P2−P1=I∫y1y2R(y)dy  (6)
Inventors have observed that by taking the difference of the two signals, measurements may be influenced by light intensity fluctuations.
The ever increasing drawing speeds in optical fibre manufacturing require an accurate and continuous monitoring of the tension, especially if the process involves an essentially constant speed during drawing. Nowadays, manufacturing of dispersion-shifted or non-zero dispersion (NZD) fibres are often carried out at drawing speeds exceeding 10 m/s and pulling tensions of 200 g or more. Special fibres, such as Raman fibres, dispersion-compensating fibres or high numerical aperture fibres, can be drawn at even larger tension, e.g., 300–400 g.